Thickness measurement is an appropriately usable checking measurement with respect to printed products or articles. It is advantageously carried out at the start or finish of processing stages or on conveying means. It can e.g. be established whether at a specific point of the processing sequence the correct number (e.g. 1) of successive printed products is being conveyed or whether individually conveyed printed products have the correct number of pages, i.e. whether they are complete and possibly also whether they contain folded or otherwise damaged pages. It is important to detect as rapidly as possible irregularities in the continuous feed flow or delivery and remove as early as possible from the processing sequence incomplete or damaged copies. When processing at high speeds, irregularities in the feed or conveying flow not only lead to faulty products, but also, if introduced into processing stages, can cause production interruptions or even damage to machinery. Further processing of damaged or incomplete copies reduces the production capacity, increases the amount of waste and the probability of delivering inadequate products. It is also possible to systematically check and control the participating machinery through checks on products between the individual processing stages.
Apparatuses for measuring the thickness of printed products are e.g. known from U.S. Pat. No. 4,560,159 assigned to Applicant's assignee and the corresponding Swiss patent 660 350 and as well as DE-OS 39 13 740 and DE-OS 38 23 201. The printed products are passed in a thickness measurement station between a reference part, e.g. a fixed roll or roller, and a deflectable measurement part, e.g. a deflectable rotary roll or roller, the reference part being fitted on one side of the conveyed flow and the measurement part on the other side. In the inoperative position the two parts have a spacing from one another which is smaller than the thickness of the flow. The deflection of the measurement part is measured and interrogated in a time cycle corresponding to that of the printed products conveyed through the measurement station. Such prior art apparatuses are suitable for checking the regularity of the conveying flow and/or for checking the individual copies, i.e. it can detect a double copy in a flow of individual copies, or also a copy having a thickness not corresponding to the desired thickness. The measured result can be easily evaluated if it is a flow of successive, individual copies or a scale flow with a large scale spacing. In the case of a scale flow with a large scale spacing the individual copies are so far apart that only the edges of each copy rest on the preceding or following copy, whilst the central part of each copy is free, i.e. the scale spacing A (distance between an edge of a copy and the corresponding edge of the following copy) is greater than half the length L of a copy in the conveying direction, i.e. A&gt;L/2. Thus, for each copy there is a point at which the scale flow thickness corresponds to the copy thickness. However, it is only in such a scale flow with a large scale spacing that the prior art apparatuses are able to directly measure the thickness of each individual printed product in the scale flow.
However, if printed products are conveyed in a scale flow with a small scale spacing, i.e. in a scale flow in which the scale spacing A is equal to or smaller than half the copy length L (A.ltoreq.L/2), so that at each point of the scale flow several printed products rest on one another, although the described apparatuses which measure the thickness of the entire flow can still establish errors in the organization of the scale flow and errors on copies, they are unable to establish which of the measured copies is faulty, because in all cases several copies are measured together. The faulty copy can only be established with considerable calculation expenditure. In particular when using the prior art measuring apparatuses, special evaluation methods are required for the start and finish of such a scale flow, because at these points it has thicknesses which are not "normal".
It would be desirable to have a method and/or an apparatus enabling the thickness of flat articles, e.g. printed products, which are moved in a flow, particularly in a scale flow with small scale spacings (A.ltoreq.L/2) to be measured and, specifically, the thickness of individual elements or the thickness of each individual element of the scale flow. For thickness measurement purposes it should not be necessary to reorganize and stop the scale flow. The evaluation of the measured results should always be the same for a continuous scale flow, particularly for its starting and finishing portions. The results of the thickness measurement must be usable for controlling means for ejecting damaged or incomplete copies and parts of the scale flow containing organizational errors. Apart from the checking of scale flows with a small scale spacing, the method and apparatus must also be usable for checking scale flows with a large scale spacing and individually conveyed printed products. The method and apparatus must be usable at a maximum number of different feed or conveying flow locations.